So, if I know my 1RM, is there an easy way for me to calculate where I should start my weights for GSLP?

don't even worry about it. start too light. on the last set you will be doing as many as possible so no matter the weight you will get a good workout. these high rep sets so good things. if you add weight like you are suppose to then you will find it getting heavy soon enough. and in a matter of a few months you will have to reset. whether it takes 3 or 6 months to reset is not going to make a difference in the long run and grand scheme of things. where you will be a year from now will be stronger either way. as JP says, don't be in a hurry to hit walls. 20 reps on the squat, bench, or press will do wonders for your mental toughness and growth. if you find yourself hitting that high of reps then just make a bigger weight jump on the next workout, like adding 10lbs instead of 5 to the squat or 5lbs instead of 2.5 to the bench or press. if you still high high reps, then keep making bigger jumps until your reps are down to the 12 range, and then start making the normal jumps. the only exception (at least for me) is the Deadlift. I never like to go super high reps. if you get to 10 reps stop. make a bigger jump the next workout until you get to 10 reps or less. then make the normal jumps each week. that is my advise anyway.

In the GSLP AMRAP sets. Should I be going until I fail regardless of form or to the point where my form breaks down?

For example today, I hit 13 reps on my last squat set because my form started going to crap. I coul have probably grinded out 2 more but decided against it because my butt kept shooting out during the lift.

I'm assuming to go until form falls apart, but I just want to clarify.

That's part of the reason I'm going to do this program. I like the rep range for getting strong, but also for cutting fat (without that being the main focus). I would like to start it for sure next monday (had some experimentation with the weights a few weeks ago).

Neck work isn't necessary at all, and you only need to replace it with something if that something else fits your goals. But yeah, something like heavy farmers walks makes for a good finisher, and heavy DB rows for sets of 20-30 will do good things for your grip while also helping to improve your upper back shelf for squats.